Method and apparatus for producing ethylene-vinyl acetate copolymer saponification product

ABSTRACT

A method for producing a saponified ethylene-vinyl acetate copolymer (EVOH) comprises: supplying into a second reactor a solution containing the copolymer that is saponified partially with an alkali catalyst under a predetermined pressure in a first reactor; and saponifying further the partially-saponified copolymer in the second reactor under a pressure higher than the predetermined pressure in the first reactor. According to the method, an EVOH having a high saponification degree can be produced efficiently with a small amount of alkali catalyst.

TECHNICAL FIELD

[0001] The present invention relates to a method and an apparatus forproducing a saponified ethylene-vinyl acetate copolymer (hereinafterabbreviated as “EVOH”).

BACKGROUND ART

[0002] In production of an EVOH by saponification of an ethylene-vinylacetate copolymer (hereinafter abbreviated as “EVAc”), it has been anobject for a person skilled in the art to produce efficiently an EVOHhaving a high saponification degree in a short time. For this object,various methods have been proposed. Specifically, known methods includea method of saponifying EVAc in a methanol solution in the presence ofan alkali catalyst in a tower reactor (Japanese Published ExaminedPatent Application No. Sho 43(1968)-14958); a method of saponifying anEVAc in a methanol solution in the presence of an alkali catalyst andfurther saponifying (re-saponifying) the EVAc in an aqueous solution ofat least 50° C. in the presence of an alkali catalyst (JapanesePublished Examined Patent Application No. Sho 45(1970)-34152); and amethod using a mixture of a water-soluble lower alcohol and water as asolvent for re-saponification (Japanese Published Unexamined PatentApplication/JP-A No. Sho 48(1973)-43493. In the methods includingre-saponification, a partially-saponified product separated from areaction solution by means of centrifugation or the like is introducedinto a separate solvent in which saponification is carried out again.

[0003] An alkali catalyst used for saponification of EVAc may causedegradation of the polymer such as caustic-soda corrosion or coloring.An acetic acid compound as a by-product remaining in the product willreduce the thermal stability, and cause coloring and generation of gelsand hard spots. It is therefore desirable to decrease the amount of thealkali catalyst used. However, since simply decreasing the amount of thealkali catalyst used will lower the rate of the saponification in theabove-described conventional methods, this decrease in the amount usedshould be limited.

[0004] Since the solubility of an EVAc having a low ethylene content ina saponification solvent such as methanol decreases as thesaponification proceeds, a reaction under conditions of high temperatureand high pressure temperature and high pressure apparently increases theabove-described by-product, coloring and generation of hard spots.

DISCLOSURE OF INVENTION

[0005] After keen studies in view of the above issues, the inventorsfound that an EVOH having a high saponification degree can be producedefficiently with an alkali catalyst used in a smaller amount whencompared to conventional methods, by selecting reaction conditionssuitable to respective stages during saponification corresponding todifferences in reaction mechanisms between initial stages and finalstages of EVAc saponification, and to variations in solubility of theEVOH with respect to the solvent depending on the progress ofsaponification. While this method is applicable regardless of theethylene content, it is particularly effective when applied to an EVOHhaving a small ethylene content.

[0006] Specifically, the present invention provides a method forproducing an EVOH by saponification in a methanol solution containing analkali catalyst, the method including supplying a solution containing apartially-saponified product of EVAc, obtained by saponification in afirst reactor under a predetermined pressure, from the first reactor toa second reactor, and saponifying further the partially-saponifiedproduct in the second reactor under a pressure higher than thepredetermined pressure in the first reactor so as to provide a finalsaponified EVAc product.

[0007] According to the method of the present invention, consumption ofan alkali catalyst is suppressed in an initial stage of saponificationof EVAc, and the saponification can proceed even in a final stage of thesaponification. This method does not require an operation to separate apartially-saponified product from a reaction solution and introducingthis partially-saponified product into a separate solvent. Therefore, anEVOH having a high saponification degree can be produced efficientlywith a small amount of alkali catalyst.

[0008] It is preferable in the method that the ethylene content of theEVAc is from 10 mol % to 60 mol %, and more preferably, from 10 mol % to30 mol %. As mentioned above, saponification of an EVAc having a lowerethylene content according to a conventional method will cause problemsof by-products such as acetic acid compound, coloring and generation ofhard spots. Since this method requires less alkali catalyst, theseproblems can be avoided easily.

[0009] In the method of the present invention, preferably a pressure inthe first reactor is from 0.1 MPa to 0.2 MPa, and a pressure in thesecond reactor is from 0.2 MPa to 2.0 MPa. A preferable saponificationdegree of the partially-saponified product to be supplied to the secondreactor is from 60 mol % to 98 mol %. It is preferable that thesaponification degree of the final saponified product is raised to 99.0mol % or higher.

[0010] In the method of the present invention, an alkali catalyst can beused in a range from 0.01 to 0.03 in molar ratio to an acetyl group inthe EVAc. If the amount of the alkali catalyst is excessive, visibleimperfections such as coloring, generation of gels and hard spots andfish-eyes might occur.

[0011] The alkali catalyst can be supplied to the first reactor alone,or it can be supplied to both the first reactor and the second reactor.Supplying to only the first reactor is advantageous in ease ofcontrolling the total amount of the alkali catalyst used. Supplying toboth the first and second reactors is advantageous in reducing the totalamount of the alkali catalyst used.

[0012] When supplying the alkali catalyst to the first reactor alone, itis preferable that a solution supplied from the first reactor to thesecond reactor contains the alkali catalyst in a range from 0.005 to0.03 in molar ratio to an acetyl group contained in thepartially-saponified product. An excessive amount of alkali catalyst maylead to the above-described imperfections. On the contrary, when theamount is insufficient, saponification in the second reactor may notproceed sufficiently.

[0013] When the alkali catalyst is supplied to the second reactor aswell, the second reactor can be a tower reactor that is divided into atleast two regions in a direction from top to bottom thereof, and thealkali catalyst is supplied to the respective regions. Accordingly, thetotal amount of the alkali catalyst used can be reduced by separatelysupplying the catalyst to the second reactor.

[0014] It is preferable in the method of the present invention that thefirst reactor and the second reactor are tower reactors. By using towerreactors, saponification of the EVAc can be carried out continuously andefficiently. In this case, a methanol solution of an EVAc and a methanolsolution of an alkali catalyst can be supplied through an upper portionof the first reactor, and then, a methanol solution containing apartially-saponified product of the EVAc and the alkali catalyst can besupplied from a lower portion of the first reactor to an upper portionof the second reactor. A methanol solution containing a final saponifiedproduct is drawn out from a lower portion of the second reactor.Consumption of the alkali catalyst can be reduced further by supplying amethanol vapor from at least one of the lower portions of the firstreactor and the second reactor, and by saponifying while discharging themethanol vapor, along with methyl acetate as a by-product of thesaponification, from the upper portion of the reactor to which themethanol vapor is supplied.

[0015] An apparatus for producing EVOH in accordance with the presentinvention is an apparatus for saponifying EVAc in a methanol solutioncontaining an alkali catalyst, and it includes a first reactor having acopolymer supplying pipe through which a methanol solution of EVAc isintroduced and a catalyst supplying pipe through which the alkalicatalyst is introduced; a second reactor having a final saponifiedproduct discharging pipe through which a final saponified product ofEVAc is drawn out; a partially-saponified product delivering pipe thatconnects the first reactor and the second reactor so as to allow asolution to be delivered and supply the partially-saponified product ofEVAc from the first reactor to the second reactor; and apressure-regulating valve for regulating the pressure in the secondreactor.

[0016] The method of the present invention can be carried outcontinuously using the apparatus of the present invention. Thisapparatus is suitable for industrial production of EVOH having a highsaponification degree, using a small amount of alkali catalyst.

[0017] For the apparatus of the present invention, the first reactor andthe second reactor can be tower reactors, where a copolymer supplyingpipe and a catalyst supplying pipe are connected to the upper portion ofthe first reactor, and a final saponified product discharging pipe isconnected to the lower portion of the second reactor, and the firstreactor is connected at the lower portion to the upper portion of thesecond reactor via a partially-saponified product delivering pipe. Inthis case, a methanol vapor supplying pipe can be connected to the lowerportion of at least one of the first and second reactors, and a methylacetate discharging pipe can be connected to the upper portion of the atleast one reactor.

BRIEF DESCRIPTION OF DRAWING

[0018]FIG. 1 shows a configuration of an embodiment of a producingapparatus according to the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

[0019] In the following, a preferred embodiment of the present inventionis described in detail.

[0020] An EVOH is difficult to dissolve in methanol especially when theethylene content is low. For example, an EVOH having an ethylene contentof about 50 mol % or less and a saponification degree of at least 99.3mol % does not dissolve in methanol substantially at or below theboiling point of methanol under atmospheric pressure. By raisingpressure in the saponification reactor and by raising the temperature ofthe methanol to dissolve the EVOH, a methanol solution of EVOH having asaponification degree of at least 99.3 mol % can be obtained. On theother hand, the saponification under high pressure and high temperaturewill also accelerate consumption of the alkali catalyst, which is causedby methyl acetate as a by-product of the saponification.

[0021] The EVOH's solubility with respect to methanol is lowered assaponification proceeds. Therefore, in an initial stage ofsaponification, the EVOH can be saponified while it is still dissolvedin methanol even under a relatively low pressure (e.g., an atmosphericpressure). Considering this, an EVOH having a high saponification degreecan be obtained while suppressing consumption of an alkali catalyst,first by carrying out saponification under a relatively low pressure andremoving methyl acetate as a by-product, and subsequently continuing thesaponification under a relatively high pressure after the solubility ofEVOH with respect to methanol is lowered as saponification proceeds. Thesaponification with a small amount of alkali catalyst can decreasecoloring or the like in an EVOH final product.

[0022] Saponification in the initial stage is mainly transesterificationbetween EVAc (or EVOH having a low saponification degree) and methanol.This reaction proceeds as a chain-reaction even under atmosphericpressure since substituted hydroxyl groups accelerate release ofadjacent acetyl groups. However, in an EVOH having copolymerizedethylene, improvement in the saponification degree provided by thisreaction alone is limited. For this reason, a direct saponification willbe required for saponifying further a partially-saponified product thathas obtained a comparatively high saponification degree. By keeping themethanol at a high temperature and by raising the pressure in a stagewhere a direct saponification is required, EVOH solubility with respectto the methanol is maintained and also saponification rate can beincreased with a minimum increase in alkali concentration. Since theamount of vinyl acetate remaining in a copolymer is small in a stagewhere saponification has proceeded to some degree, consumption of alkalicatalyst is not increased drastically even when the conditions becomeharsh to accelerate the reaction.

[0023] As mentioned above, by adjusting the pressure to be relativelyhigh as saponification proceeds, an EVOH having a high saponificationdegree can be produced with an extreme efficiency, taking into accountthe difference of reaction mechanism between initial stages and finalstages of saponification and the variation in solubility of EVOH withrespect to methanol, where the variation is caused by the progress ofsaponification.

[0024] Since an EVAc can be saponified substantially to a finalsaponified product in a single solvent according to this method, thereis no need for preparing a separate solvent (e.g., an aqueous solutionfor direct saponification) or conducting filtration or centrifugationfor transferring the EVOH to the separate solvent. Therefore,saponification of the EVAc can be carried out continuously using pluralreactors with adjusted interior pressures.

[0025] Though there is no specific limitation on the ethylene content ofthe EVAc to be supplied, a preferable range is from 10 mol % to 60 mol%, and more preferably from 10 mol % to 30 mol %. In a comparison withan ordinary saponification method, the improvement becomes remarkable,especially when saponifying an EVAc having an ethylene content in arange from 10 mol % to 30 mol %.

[0026] An example of a saponification apparatus comprising a pluralityof reactors will be described below referring to FIG. 1. In thissaponification apparatus, an EVAc supplying pipe 12 and an alkalicatalyst supplying pipe 13 are connected to an upper portion of a firstreactor 1, and a methanol vapor supplying pipe 15 is connected to alower portion thereof. A methyl acetate discharging pipe 11 and apartially-saponified product delivering pipe 16 are connectedrespectively to a top portion and a bottom portion of the first reactor.This pipe 16 is linked to an upper portion of a second reactor 2 via apump 36 so as to deliver a solution containing a partially-saponifiedproduct to the second reactor 2.

[0027] A methyl acetate discharging pipe 21, an alkali catalystsupplying pipe 23 and a methanol vapor supplying pipe 25 are connectedto the portions of the second reactor 2 that correspond to therespective portions of the first reactor. A final saponified productdischarging pipe 26 is connected to the bottom portion of the secondreactor so as to draw out a methanol solution containing a finalsaponified product.

[0028] Alkali catalyst supplying pipes 14 and 24 are connected as wellto middle portions of the first reactor and second reactor so as tosupply a catalyst from plural positions.

[0029] The reactors 1 and 2 are not connected to the outside except viathe respective pipes, so that they are substantially sealed when thepipes are closed. Therefore, pressures in the reactors can be adjustedto predetermined levels by adjusting valves 31 and 41 arranged at themethyl acetate discharging pipes 11 and 21. Alternatively, such apressure-regulating valve can be equipped to the second reactor alone.

[0030] The following is an example of a method for continuoussaponification of EVAc using this saponification apparatus. In thismethod, sodium hydroxide is used as an alkali catalyst.

[0031] To the upper portion of the first reactor 1, a methanol solutionof EVAc (EVAc/MeOH) and a methanol solution of sodium hydroxide(NaOH/MeOH) are supplied through pipes 12 and 13 respectively. Forpreventing the solution viscosity in the reactor from risingexcessively, the EVAc concentration in the supplied methanol solution ispreferably in a range from 30 wt % to 60 wt %. The amount of sodiumhydroxide to be supplied can be determined suitably so as to correspondto the ethylene content of the EVAc, the desired saponification degreeor the like. The following description refers to an example in which thealkali catalyst is supplied to the second reactor as well. When sodiumhydroxide is supplied to the first reactor alone, the amount is adjustedso that hydroxyl groups will remain in the solution in the deliveringpipe 16 in a range from 0.005 to 0.03 in molar ratio to an acetyl groupcontained in the partially-saponified product.

[0032] A methanol (eOH) vapor is blown into the lower portion of thetower through the pipe 15. The amount of the methanol vapor to be blownis preferably selected from a range such that the methyl acetateby-product can be removed almost completely. For example, the amount canbe from 1 weight part to 10 weight parts with respect to 1 weight partof EVAc. The temperature of the methanol vapor can be set as the boilingpoint of methanol at the pressure in the reactor.

[0033] From the top portion of the first reactor, the methanol (MeOH)vapor as well as methyl acetate (MeOAc) by-product are discharged. Sucha stripping of methyl acetate by blowing of a methanol vapor iseffective in suppressing consumption of the alkali catalyst and inincreasing the saponification rate. Preferable pressure inside the firstreactor is from 0.1 MPa to 0.2 MPa, and especially preferably from 0.1MPa to 0.15 MPa, or it can be an atmospheric pressure. Excessive rise intemperature of the methanol solution in this stage will accelerate areaction between the methyl acetate and sodium hydroxide. However, sincean EVOH having a low saponification degree has a high solubility withrespect to methanol, there is no need to raise the temperature of themethanol solution.

[0034] A methanol solution containing a partially-saponified productgenerated due to saponification in the first reactor is delivered to thesecond reactor 2 by use of pump 36. This partially-saponified producthas a saponification degree ranging from 60 mol % to 98 mol %,especially preferably, from 80 mol % to 95 mol %. If the saponificationproceeds excessively, solubility of the EVOH is lowered excessively andpolymer scales will adhere to the reactors and pipes, which may causedifficulty in a long-term continuous operation. If the saponificationdegree is suppressed excessively, the amount of the alkali catalystrequired cannot be reduced sufficiently.

[0035] The partially-saponified product supplied to the second reactor 2is saponified further in the reactor. Similar to the first reactor, thissecond reactor is supplied with a methanol solution of sodium hydroxide(NaOH/MeOH) and a methanol (MeOH) vapor respectively through the upperportion and the lower portion thereof. Similarly in this reactor, theEVOH is saponified further while methyl acetate (MeOAc) is dischargedfrom the top portion of the reactor.

[0036] The pressure range in the second reactor should be determined tobe higher than in the first reactor. Since the solubility of thepartially-saponified product with respect to methanol depends on theethylene content, the partially-saponified product may become insolublein methanol when the ethylene content is low. Therefore, it ispreferable that the pressure is suitably selected corresponding to theethylene content of the EVOH to be produced. Preferably, the pressure inthe second reactor is from 0.2 MPa to 2.0 MPa, more preferably from 0.2MPa to 1.0 MPa, and further preferably from 0.2 MPa to 0.5 MPa.

[0037] For the apparatus shown in the drawing, the pressures in thereactors are adjusted mainly by opening and closing the valves 31 and41. Since the amount of methyl acetate formed as a by-product in thesecond reactor is less than that formed in the first reactor, theby-product can be removed sufficiently even when the discharge amountfrom the pipe 21 is limited due to raising the pressure in the interiorsof the reactors. For example, the amount of a methanol vapor blown intothe second reactor is preferably about from 0.1 weight parts to 2 weightparts with respect to 1 weight part of the partially-saponified product.Similarly, the temperature of the methanol vapor can be the boilingpoint of methanol under the pressure within the second reactor.

[0038] In this way, a final saponified product that has been saponifiedfurther in the second reactor is taken out from the bottom portion ofthis reactor. The final saponified product has a saponification degreeof at least 99.0 mol %, or preferably, 99.3 mol %.

[0039] According to the above-described method, an EVAc can besaponified continuously under reaction conditions adjusted correspondingto the respective stages in the saponification. Moreover, thesaponification can be carried out as a homogeneous reaction in general,and thus the above-described method is advantageous from the viewpointof obtaining an increased reaction speed.

[0040] The methanol solution of sodium hydroxide can be supplied intothe towers from the middle portions of the towers through the pipes 14and 24 as well as from the upper portions. Moreover, the supplyingsections can be divided further. By supplying an alkali catalyst fromtwo or more sections, the total amount of the alkali catalyst used canbe reduced, and coloring or the like of the final product can bedecreased. Divided supplying of the alkali catalyst is particularlyeffective for the second reactor.

[0041] The present invention is not limited to the above-exemplifiedapparatus and method for using two reactors, but alternatively three ormore reactors connected to allow a solution to be delivered can be used.When three or more reactors are used, the above-exemplifiedconfiguration can be applied to the two reactors during the last stage.In the above embodiment, an EVAc is supplied to the first reactor. In acase in which three or more reactors are used, the reactor used as thefirst reactor above is supplied not with EVAc but EVOH that has beenpartially-saponified in a reactor of a foregoing stage.

[0042] Sodium hydroxide exemplified above as the alkali catalyst can bereplaced by potassium hydroxide and alkaline metal alcoholate (e.g.sodium methylate) or the like. Any methanol solutions containingmethanol as the main component can be used, and such methanol solutionscan contain other alcohols having 2-4 carbon atoms, such as ethanol,n-propanol, i-propanol, n-butanol, and t-butanol.

[0043] The EVAc to be saponified can be produced by copolymerizingethylene and vinyl acetate according to a conventional method. Althoughthere is no limitation to the polymerization method, solvent, etc.,solution polymerization using methanol as a solvent is suitable. As apolymerization catalyst, a radical initiator, e.g., various types ofazonitrile-based initiators or organic peroxide-based initiators, may beused. Furthermore, a third monomer (e.g. α-olefins such as propylene,unsaturated acids such as acrylic acid, various kinds of nitrites,various kinds of amides) copolymerizable with ethylene and vinyl acetatemay also be present in the EVAc. It is preferable that the third monomeris present in an amount which does not inhibit the effect of the presentinvention.

[0044] It is preferable that the melt index (MI) of the EVOH obtained asthe final saponified product is from 0.1 to 200 g/10 min. As the MI, avalue measured at 190° C. and under a load of 2160 g is used. However,with respect to an EVOH having a melting point of around 190° C. orexceeding 190° C., a plurality of measured values under theabove-specified load and at temperatures of not less than the meltingpoint are plotted as a semi-logarithmic graph with the inverse of theabsolute temperature on the horizontal axis and MI on the vertical axis(logarithmic scale), and a value extrapolated to 190° C. is used as theMI.

[0045] Usually, the EVOH obtained by saponification is extruded furtherinto a coagulation bath including water or a mixture of water andmethanol, and cut into pellets. These pellets are washed, deliquored,and treated with a boron compound, a carboxylic acid compound, aphosphoric acid compound, etc. as appropriate. By including thesecompounds, mechanical properties, thermal stability, and the like ofEVOH molded products can be improved.

[0046] The thus obtained EVOH may be molded into various forms such asfilms, sheets, containers, pipes, fibers and the like by melt molding.Melt molding may be effected by extrusion molding, inflation, blowmolding, melt spinning, injection molding, etc. It is preferable thatthe melting temperature is from 150° C. to 270° C. A blend of at leasttwo kinds of EVOH different in polymerization degree, ethylene content,saponification degree and the like may be melt molded. Furthermore,plasticizers, stabilizers, surfactants, crosslinking agents, metalsalts, fillers, reinforcing agents such as various types of fibers, etc.may be added to the EVOH in advance.

[0047] A thermoplastic resin other than EVOH may be mixed with the EVOH.Examples of the thermoplastic resin include polyolefins (polyethylene,polypropylene, poly-1-butene, poly-4-methyl-1-pentene,ethylene-propylene copolymers, copolymers of ethylene and an α-olefinhaving at least 4 carbon atoms, copolymers of a polyolefin and maleicanhydride, EVAc, ethylene-acrylic acid ester copolymers, modifiedpolyolefins in which these polyolefins are grafted with an unsaturatedcarboxylic acid or its derivative, etc.), various types of nylons (nylon6, nylon 66, nylon 6/nylon 66 copolymers, etc.), polyvinyl chloride,polyvinylidene chloride, polyester, polystyrene, polyacrylonitrile,polyurethane, polyacetal, modified polyvinyl alcohol resin, and thelike.

[0048] Furthermore, the EVOH may be formed into a laminate, for example,by co-extruding the EVOH with a thermoplastic resin such as thoseexemplified above. Furthermore, the EVOH may be formed into a laminatewith a substrate film such as paper, plastic film, metal foil or thelike, and may be coated on the surface of these substrate films byco-extrusion coating, solution coating or the like.

EXAMPLES

[0049] In the following, the present invention will be described in moredetail with reference to the following non-limiting examples.

Example 1

[0050] An EVAc having an ethylene content of 44 mol % was dissolved inmethanol to obtain a solution of 46 wt %. Next, the solution wassupplied at 16 g/min. and a 5 wt % methanol solution of sodium hydroxidewas supplied at 0.9 g/min. through an upper portion of a filling tower(hereinafter abbreviated as a tower reactor A) having a diameter of 100mm in which Raschig rings having diameters of 15 mm was filled over afilling length of 520 mm. A molar ratio of the sodium hydroxide to avinyl acetate unit in the EVAc was 0.017. Methanol vapor was blown intothe tower reactor at 58 g/min. through a bottom portion, and methylacetate formed as a by-product was distilled as a mixture with excessmethanol through a top portion thereof. The temperature in the tower was66° C., the pressure was about 0.1 MPa (substantially atmosphericpressure), and the average residence time of the EVAc in the tower wasabout 20 minutes. As a result of this reaction, a methanol solution(copolymer concentration: 23 wt %) of a partially-saponified EVOH havinga saponification degree of 90.8 mol % was obtained at a rate of 21g/min. In this solution, 0.14 wt % of sodium hydroxide remained.

[0051] The above-described methanol solution of the partially-saponifiedEVOH was supplied at 21 g/min. through an upper portion of a fillingtower (hereinafter abbreviated as a tower reactor B) having a diameterof 150 mm in which Raschig rings having diameters of 12 mm was filledover a filling length of 770 mm, methanol vapor was blown into the towerreactor at 24 g/min. through a bottom portion, and methyl acetate formedas a by-product was distilled as a mixture with excess methanol througha top portion thereof. The temperature in the tower was 104° C., thepressure was about 0.3 MPa, and the average residence time of the EVOHin the tower was about 40 minutes. As a result of this reaction, amethanol solution (copolymer concentration: 22 wt %) of an EVOH having asaponification degree of 99.4 mol % was obtained at a rate of 20 g/min.

[0052] To the thus obtained methanol solution of EVOH, acetic acid wasadded at a ratio of 2 weight parts to 100 weight parts of the EVOH so asto neutralize the remaining sodium hydroxide. The neutralized methanolsolution contained sodium acetate at a ratio of 1.85 weight parts withrespect to 100 weight parts of the EVOH. Next, the solution wasconcentrated until the copolymer concentration became 45 wt %. Water wasadded at a ratio of 24 weight parts to 100 weight parts of the solutionand the temperature was raised to 60° C. so that a homogeneous solutionwas obtained. This solution was extruded at a rate of 15 g/min. in amethanol-water mixed solvent (methanol/water=10/90, weight ratio) keptat 5° C. through a nozzle having a bore diameter of 2 mm and coagulatedin the form of strands. The strands were cut by a cutter to obtainpellets with a length of 2.7-3.0 mm. Furthermore, these pellets wereintroduced into a large volume of 0.1 g/L aqueous solution of aceticacid and washed to remove remaining methanol and sodium acetate.Subsequently, the pellets were dried for 5 hours at 60° C., and furtherdried for 10 hours at 110° C., so that EVOH dry pellets having anethylene content of 44 mol % and a saponification degree of 99.4 mol %were obtained. The pellets were colorless and transparent. The resultsare shown in Table 1.

Example 2

[0053] An EVAc having an ethylene content of 27 mol % was dissolved inmethanol to obtain a solution of 46 wt %. Next, the solution wassupplied at 16 g/min. and a 5 wt % methanol solution of sodium hydroxidewas supplied at 0.7 g/min. through an upper portion of a tower reactorA. A molar ratio of the sodium hydroxide to a vinyl acetate unit in theEVAc was 0.012. Methanol vapor was blown into the tower reactor at 67g/min. through a bottom portion, and methyl acetate formed as aby-product was distilled as a mixture with excess methanol through a topportion thereof. The temperature in the tower was 65° C., the pressurewas about 0.1 MPa (substantially atmospheric pressure), and the averageresidence time of the EVAc in the tower was about 20 minutes. As aresult of this reaction, a methanol solution (copolymer concentration:22 wt %) of a partially-saponified EVOH having a saponification degreeof 89.5 mol % was obtained at a rate of 20 g/min. In this solution, 0.09wt % of sodium hydroxide remained.

[0054] The above-described methanol solution of partially-saponifiedEVOH was supplied at 20 g/min. through an upper portion of a towerreactor B, methanol vapor was blown into the tower reactor at 24 g/min.through a bottom portion, and methyl acetate formed as a by-product wasdistilled as a mixture with excess methanol through a top portionthereof. The temperature in the tower was 103° C., the pressure wasabout 0.3 MPa, and the average residence time of the EVOH in the towerwas about 40 minutes. As a result of this reaction, a methanol solution(copolymer concentration: 21 wt %) of an EVOH having a saponificationdegree of 99.3 mol % was obtained at a rate of 20 g/min.

[0055] To the thus obtained methanol solution of EVOH, acetic acid wasadded at a ratio of 2 weight parts to 100 weight parts of the EVOH so asto neutralize the remaining sodium hydroxide. The neutralized methanolsolution contained sodium acetate at a ratio of 1.60 weight parts withrespect to 100 weight parts of the EVOH. Next, the solution wassubjected to concentration, coagulation, washing and drying as describedin Example 1, so that EVOH dry pellets having an ethylene content of 27mol % and a saponification degree of 99.3 mol % were obtained. Thepellets were colorless and transparent. The results are shown in Table1.

Example 3

[0056] An EVAc having an ethylene content of 35 mol % was dissolved inmethanol to obtain a solution of 46 wt %. Next, the solution wassupplied at 16 g/min. and a 5 wt % methanol solution of sodium hydroxidewas supplied at 0.8 g/min. through an upper portion of a tower reactorA. The molar ratio of the sodium hydroxide to a vinyl acetate unit inthe EVAc was 0.014. Methanol vapor was blown into the tower reactor at58 g/min. through a bottom portion, and methyl acetate formed as aby-product was distilled as a mixture with excess methanol through a topportion thereof. The temperature in the tower was 66° C., the pressurewas about 0.1 MPa (substantially atmospheric pressure), and the averageresidence time of the EVAc in the tower was about 20 minutes. As aresult of this reaction, a methanol solution (copolymer concentration:23 wt %) of a partially-saponified EVOH having a saponification degreeof 91.1 mol % was obtained at a rate of 20 g/min. In this solution, 0.09wt % of sodium hydroxide remained.

[0057] The above-described methanol solution of partially-saponifiedEVOH was supplied at 20 g/min. through an upper portion of a towerreactor B, methanol vapor was blown into the tower reactor at 24 g/min.through a bottom portion, and methyl acetate formed as a by-product wasdistilled as a mixture with excess methanol through a top portionthereof. The temperature in the tower was 103° C., the pressure wasabout 0.3 MPa, and the average residence time of the EVOH in the towerwas about 40 minutes. As a result of this reaction, a methanol solution(copolymer concentration: 22 wt %) of an EVOH having a saponificationdegree of 99.3 mol % was obtained at a rate of 20 g/min.

[0058] To the thus obtained methanol solution of EVOH, acetic acid wasadded at a ratio of 2 weight parts to 100 weight parts of the EVOH so asto neutralize the remaining sodium hydroxide. The neutralized methanolsolution contained sodium acetate at a ratio of 1.70 weight parts withrespect to 100 weight parts of the EVOH. Next, the solution wassubjected to concentration, coagulation, washing and drying as describedin Example 1, so that EVOH dry pellets having an ethylene content of 35mol % and a saponification degree of 99.3 mol % were obtained. Thepellets were colorless and transparent. The results are shown in Table1.

Comparative Example 1

[0059] An EVAc having an ethylene content of 44 mol % was dissolved inmethanol to obtain a solution of 46 wt %. Next, the solution wassupplied at 16 g/min. and a 5 wt % methanol solution of sodium hydroxidewas supplied at 3.6 g/min. through an upper portion of a tower reactorB. The molar ratio of the sodium hydroxide to a vinyl acetate unit inthe EVAc was 0.066. Methanol vapor was blown into the tower reactor at58 g/min. through a bottom portion, and methyl acetate formed as aby-product was distilled as a mixture with excess methanol through a topportion thereof. The temperature in the tower was 104° C., the pressurewas about 0.3 MPa, and the average residence time of the EVOH in thetower was about 40 minutes. As a result of this reaction, a methanolsolution (copolymer concentration: 23 wt %) of an EVOH having asaponification degree of 99.4 mol % was obtained at a rate of 20 g/min.

[0060] To the thus obtained methanol solution of EVOH, acetic acid wasadded at a ratio of 2 weight parts to 100 weight parts of the EVOH so asto neutralize the remaining sodium hydroxide. The neutralized methanolsolution contained sodium acetate at a ratio of 7.40 weight parts withrespect to 100 weight parts of the EVOH. Next, the solution wassubjected to concentration, coagulation, washing and drying as describedin Example 1, so that EVOH dry pellets having an ethylene content of 44mol % and a saponification degree of 99.4 mol % were obtained. Thepellets were yellow. The results are shown in Table 1.

Comparative Example 2

[0061] An EVAc having an ethylene content of 27 mol % was dissolved inmethanol to obtain a solution of 46 wt %. Next, the solution wassupplied at 16 g/min. and a 5 wt % methanol solution of sodium hydroxidewas supplied at 2.1 g/min. through an upper portion of a tower reactorB. The molar ratio of the sodium hydroxide to a vinyl acetate unit inthe EVAc was 0.034. Methanol vapor was blown into the tower reactor at58 g/min. through a bottom portion, and methyl acetate formed as aby-product was distilled as a mixture with excess methanol through a topportion thereof. The temperature in the tower was 104° C., the pressurewas about 0.3 MPa, and the average residence time of the EVOH in thetower was about 40 minutes. As a result of this reaction, a methanolsolution (copolymer concentration: 22 wt %) of an EVOH having asaponification degree of 99.3 mol % was obtained at a rate of 21 g/min.

[0062] To the thus obtained methanol solution of EVOH, acetic acid wasadded at a ratio of 2 weight parts to 100 weight parts of the EVOH so asto neutralize the remaining sodium hydroxide. The neutralized methanolsolution contained sodium acetate at a ratio of 4.70 weight parts withrespect to 100 weight parts of the EVOH. Next, the solution wassubjected to concentration, coagulation, washing and drying as describedin Example 1, so that EVOH dry pellets having an ethylene content of 27mol % and a saponification degree of 99.3 mol % were obtained. Thepellets were yellow. The results are shown in Table 1. TABLE 1 Alkalicatalyst By-product EVOH pellet NaOH molar NaOAc Ethylene ratio amountcontent Saponification NaOH amount (mol/VAc (g/EVOH (mol %) degree (mol%) Pellet color (g/EVAc 100 g) unit) 100 g) Examples 1 44 99.4 Colorlessand 0.61 0.017 1.85 transparent 2 27 99.3 Colorless and 0.48 0.012 1.60transparent 3 35 99.3 Colorless and 0.54 0.014 1.70 transparentComparative 1 44 99.4 Yellow 2.45 0.066 7.40 Examples 2 27 99.3 Yellow1.43 0.034 4.70

[0063] As demonstrated in the results, colorless pellets were obtainedin the respective Examples. Moreover, the amounts of the saponificationcatalysts used were reduced to about ⅓-¼ in comparison with conventionalmethods. Due to the reduction in amount of the catalysts used, labors ofwashing to remove sodium acetate as a by-product were decreased as well.

[0064] According to the present invention, an EVOH can be producedefficiently since an EVAc can be saponified at a sufficient speed for anindustrial application with a small amount of alkali catalyst. Moreover,the present invention can decrease labors of washing subsequent to areaction, and allows a stable and continuous operation. Due to decreasedamount of an alkali catalyst used, visible imperfections such ascoloring can be suppressed in an EVOH obtained according to the presentinvention.

1. A method for producing a saponified ethylene-vinyl acetate copolymerby saponifying an ethylene-vinyl acetate copolymer in a methanolsolution containing an alkali catalyst, the method comprising: supplyingfrom a first reactor into a second reactor a solution containing apartially-saponified product of the copolymer obtained throughsaponification under a predetermined pressure in the first reactor; andsaponifying further the partially-saponified product in the secondreactor under a pressure higher than the predetermined pressure in thefirst reactor, whereby to obtain a final saponified copolymer.
 2. Themethod according to claim 1, wherein an ethylene content of theethylene-vinyl acetate copolymer is from 10 mol % to 60 mol %.
 3. Themethod according to claim 2, wherein the ethylene content of theethylene-vinyl acetate copolymer is from 10 mol % to 30 mol %.
 4. Themethod according to claim 1, wherein a pressure in the first reactor isfrom 0.1 MPa to 0.2 MPa, and a pressure in the second reactor is from0.2 MPa to 2.0 MPa.
 5. The method according to claim 1, wherein thepartially-saponified product to be supplied into the second reactor hasa saponification degree ranging from 60 mol % to 98 mol %.
 6. The methodaccording to claim 1, wherein the final saponified product has asaponification degree of at least 99.0 mol %.
 7. The method according toclaim 1, wherein the amount of alkali catalyst used is from 0.01 to 0.03in molar ratio to an acetyl group contained in the ethylene-vinylacetate copolymer.
 8. The method according to claim 1, wherein thealkali catalyst is supplied to the first reactor alone.
 9. The methodaccording to claim 8, wherein the solution supplied from the firstreactor into the second reactor contains the alkali catalyst in a rangefrom 0.005 to 0.03 in molar ratio to the acetyl group contained in thepartially-saponified product.
 10. The method according to claim 1,wherein the alkali catalyst is supplied to the first reactor and thesecond reactor.
 11. The method according to claim 10, wherein the secondreactor is a tower reactor divided into at least two regions in adirection from top to bottom of the reactor, and the alkali catalyst issupplied to the respective regions.
 12. The method according to claim 1,wherein the first reactor and the second reactor are tower reactors. 13.The method according to claim 12, wherein a methanol solution of theethylene-vinyl acetate copolymer and a methanol solution of the alkalicatalyst are supplied through the upper portion of the first reactor,and a methanol solution containing the partially-saponified product ofthe copolymer and the alkali catalyst is supplied from the lower portionof the first reactor to the upper portion of the second reactor.
 14. Themethod according to claim 12, wherein a methanol vapor is suppliedthrough the lower portion of at least one reactor selected from thegroup consisting of the first reactor and the second reactor, and themethanol vapor is discharged with methyl acetate as a by-product ofsaponification from the upper portion of the reactor to which themethanol vapor is supplied.
 15. An apparatus for producing a saponifiedethylene-vinyl acetate copolymer by saponifying an ethylene-vinylacetate copolymer in a methanol solution containing an alkali catalyst,the apparatus comprises: a first reactor comprising a copolymersupplying pipe through which the methanol solution of the ethylene-vinylacetate copolymer is introduced, and a catalyst supplying pipe throughwhich the alkali catalyst is introduced; a second reactor comprising afinal saponified product discharging pipe through which a finalsaponified product of the copolymer is drawn out; a partially-saponifiedproduct delivering pipe that connects the first reactor to the secondreactor for allowing a solution to be delivered and supplying thepartially-saponified product of the copolymer from the first reactor tothe second reactor; and a pressure-regulating valve for regulatingpressure in the second reactor.
 16. The apparatus according to claim 15,wherein the first reactor and the second reactor are tower reactors, thecopolymer supplying pipe and a catalyst supplying pipe are connected tothe upper portion of the first reactor and the final saponified productdischarging pipe is connected to the lower portion of the secondreactor, and the partially-saponified product delivering pipe connectsthe lower portion of the first reactor and the upper portion of thesecond reactor.
 17. The apparatus according to claim 16, wherein amethanol vapor supplying pipe is connected to the lower portion of atleast one reactor selected from the group consisting of the firstreactor and the second reactor and a methyl acetate discharging pipe isconnected to the upper portion of said at least one reactor.